1. Technical Field
The present invention relates to a head mechanism of a thermal printer, and particularly, to a head mechanism for obtaining good recording in a thermal printer suitable for the recording that a margin is not formed at a front end in the conveying direction of a sheet.
2. Related Art
A platen roller in a head mechanism of a thermal printer can be formed from a cylindrical roller portion made of rubber, and a rotary shaft which projects from both longitudinal end surfaces of this roller portion. This rotary shaft is rotatably supported by a pair of side plates constituting a frame of the thermal printer, and is constructed so as to be rotationally driven under the transmission of the driving force of a driving motor.
A thermal head which is at a front end of a head bearing member and is always brought into pressure contact with the platen roller by the biasing force of a biasing member is arranged above the platen roller. A line-type thermal printer in which a plurality of heat-generating elements are aligned and arranged on a metallic substrate in a direction orthogonal to a conveying direction of a thermosensitive recording medium (hereinafter simply referred to as “sheet”) as a sheet provided for recording is used as this thermal head.
If the tip of a sheet to be conveyed by the driving of a sheet-conveying mechanism is inserted into between the platen roller and the thermal head, the plurality of heat-generating elements of the thermal head are made to generate heat selectively with the sheet being sandwiched with pressure contact, and the platen roller is rotated, thereby performing desired recording on the sheet (for example, refer to Japanese Unexamined Patent Application Publication Nos. 2006-142569 and 2005-280017)
However, as mentioned above, the head mechanism of the conventional thermal printer is in a state where the thermal head and the platen roller are always brought into pressure contact with each other with predetermined pressure-contact pressure by the biasing force of the biasing member even after the end of recording. Therefore, in the platen roller where the roller portion is made of rubber is deformed, the portion which the thermal head is brought into pressure contact with is deformed, and pressure-contact traces are generated in the portion. Further, blurs, streaks, etc. are generated in recording results recorded on a sheet. As a result, there is a possibility that image quality may deteriorate.
For example, in a thermal printer of a type, generally called a photograph printer, which obtains desired color recording using ink ribbons, a thermal head is brought into pressure contact with a platen roller and brought into a head-down state during recording, and the thermal head is separated from the platen roller and brought into a head-up state when recording is not performed.
As such, if this up/down movement of the thermal head is performed, the problem that the image quality resulting from the aforementioned pressure-contact traces deteriorates will not occur. However, it is necessary to provide an up/down mechanism of the thermal head to that end, and it is difficult to adopt this method in a miniaturized printer. Further, the printer which performs the up/down movement of the thermal head is constructed such that the thermal head is brought into a head-down state, and then recording is started. Therefore, there is a problem that a complicated mechanism is required in order to obtain desired recording, without providing a margin at the tip of a sheet, and consequently, the cost of parts increases.
Moreover, in the related art, the thermal printer which is constructed such that the thermal head is always brought into pressure contact with the platen roller irrespective of whether or not recording is performed as mentioned above, are often utilized for recording of recording materials which do not require high printing quality, such as receipts, bar codes, and character documents.
These and other drawbacks exist. However, with the recent increase in utilization of thermal printers which are constructed such that the thermal head is always brought into pressure contact, there is a need for improved printing quality of this thermal printer.